1. Field of the Invention
This invention relates to a machine tool with a worn-detection function for a tool, more particularly, to a machine tool in which wearing, produced one after another in process of multiple work pieces, can be detected.
2. Description of the Related Art
Conventionally, a machine tool is used for, for example, cutting various metallic materials.
The machine tool has a cutting tool attached at the end of a main shaft rotatably supported, in which the tool is rotated by rotatably driving the main shaft with a shaft driving motor, thereby performing a cutting process for a work piece.
Support for the Main Shaft
The conventional machine tool uses various bearing for rotatably supporting the main shaft.
For example, an antifriction bearing or the like is heavily used for the machine tool for cutting a heavy work piece having large cut-load.
An air static-pressure bearing is used for the machine tool for carrying out a ultra-precise cutting process of small cut-load (e.g., in the case of a mirror finish of light-metal material by using diamond bit or other cases).
Wear of the Tool
The tool used in the machine tool is worn or caused with the other damages through the cutting processes, and sometimes results in a break of the tool. And, sometimes, the aforementioned wear or the like causes precision of a processed part of the work piece to be decreased, and performance of the required process to be impossible.
Therefore, in addition to measures, for example, which an operator inspects for wear and the like on the tool as necessary, in order to attain a consecutively automatic process operation, the machine tool in which wear or the like occurred on the tool is automatically detected is proposed by the present applicant (see Japanese Patent Application Publication No. Sho 62-60221).
In the aforementioned machine tool, the worn detection has two ways of the use of current consumption of the shaft driving motor and the use of sensing vibration created in proximity to the main shaft.
Determination by Using Current Consumption
In order to determine by using current consumption, an unload current value iNUL of an unload state (in which the tool does not cut a work piece) and a normally cutting current value iNET in the use of a new tool are defined by properly detecting a current value iR of a motor, rotatably driving the main shaft. With the aforementioned current values, the increment of current (iNET-iNUL), increased when the normal cutting process is performed with the new tool, and the increment of current (iR-iNUL) in the actual cutting process are compared, thereby the wear on the tool is determined to occur when the increment of current in the actual cutting process is larger.
Each of the aforementioned current values is obtained from mean of multiple sampling values.
Determination by Sensing Vibration
In order to determine by sensing vibration, a piezoelectric element or the like is provided as a vibration detection element in proximity to the main shaft, thereby faulty process with wear or break is determined when a signal, obtained from the aforementioned element, exceeds the base value.
In the vibration determination as described above, it is required to be insensitive to various vibrations (a bearing, a motor and so on) naturally created in the machine tool. Therefore, the vibration determination uses a filter circuit for removing the aforementioned vibration components.
However, the detection of delicate wear is difficult, that is to say the vibration determination is limited to determine that chipping, break and so on occur on the tool.
Disadvantages of the Worn-detection for the Tool
The aforementioned automatic tool-worn detection is under the circumstances that the required detection of wear on the tool is insufficient.
In other words, in the aforementioned machine tool, where the bearing supporting the main shaft is an antifriction bearing, the antifriction bearing causes noise to create in a driving current detected value of the motor, so that brake of the tool or the like, which are easily represented as fluctuation, can be detected. Concerning the wear which a fluctuation signal is smaller, but, there is a disadvantage the reliable detection is difficult because of noise.
Therefore, in the conventional machine tool for cutting, the heavy work piece to require the antifriction bearing, the worn-detection for the tool is uncertain, thus producing a disadvantage an automatic operation without operators for a long time is impossible.
It is an object to provide the machine tool with the worn-detection function for the tool, which is capable of reliably detecting wear on the tool.